1. Field of the Invention
The present invention relates to a tele-side converter lens which is removably mounted to an object side of a photographing lens (master lens), which is used for a digital still camera, a video camera, a broadcast camera, and the like, so as to change a focal length of an entire system to be longer than an original focal length of the master lens.
2. Description of the Related Art
In general, as a method of changing (increasing) a focal length of the master lens (photographing lens) to a telephoto side, there is known a front teleconverter lens in which an afocal lens is removably mounted to the object side of the master lens. This method has an advantage that the focal length is changed without sacrificing (without changing) an F-number of the master lens at a telephoto end. The tele-side converter lens is one type of the teleconverter lenses, which is downsized on the precondition of being used mainly only on the telephoto side having a small angle of field.
In recent years, along with downsizing of a digital camera and a video camera and increase in number of pixels of a solid-state image pickup element such as a CCD sensor, a photographing lens used for such cameras has been required to be small in size and to have high optical performance in which a chromatic aberration is appropriately corrected. Therefore, the tele-side converter lens mounted to the photographing lens is also required to be small in size and to have high optical performance in the same manner. As this front teleconverter lens, there is known a lens including, in order from the object side to an image side, a front lens unit having a positive refractive power and a rear lens unit having a negative refractive power.
As one of such lenses, there is known a teleconverter lens formed of three lenses in which the front lens unit is formed of a single positive lens and the rear lens unit is formed of a negative lens and a positive lens (Japanese Patent Application Laid-Open No. 2003-307675). In addition, there is known a teleconverter lens formed of four lenses in which the front lens unit is formed of two positive lenses and the rear lens unit is formed of a positive lens and a negative lens (Japanese Patent Application Laid-Open No. 2005-331851). In addition, there is known a small teleconverter lens, in which each of the front lens unit and the rear lens unit is formed of a single lens (Japanese Patent Application Laid-Open No. S55-32046).
The teleconverter lens is required to have a short entire optical length (length from a first lens surface to a final lens surface), to be small in size as a whole, to have a small number of lenses for maintaining high optical performance when mounted to the master lens, and to have a small aberration variation. In general, when the teleconverter lens is mounted to the object side of the master lens so as to increase a focal length of the entire system, various aberrations such as a spherical aberration, an axial chromatic aberration, and a lateral chromatic aberration are significantly changed on the telephoto side.
An aberration amount in the teleconverter lens increases in proportion to a magnification of the teleconverter lens. Further, when the teleconverter lens is mounted to the master lens, various aberrations are increased in proportion to the changed focal length. In particular, when the teleconverter lens is mounted, in the frequently-used zoom position on the telephoto side, in terms of the axial chromatic aberration, not only primary deterioration of achromatism but also a secondary spectrum is increased and affects largely to image performance. Therefore, when mounting the teleconverter lens to a zoom lens having a high zoom ratio and a very long focal length at the telephoto end, such as a broadcast zoom lens, a generation amount of the axial chromatic aberration becomes a large problem.
In consideration of needs for high-definition images by a broadcast television camera supporting a high-definition television (HDTV) in recent years, it is particularly important to appropriately correct the axial chromatic aberration on the telephoto side so as to acquire high quality image performance.
For the reason described above, the teleconverter lens to be mounted to a broadcast zoom lens is strongly required, in addition to be small and light-weighted, to have the axial chromatic aberration appropriately corrected on the telephoto side when the teleconverter lens is mounted to the zoom lens. In particular, in order to satisfy optical performance required of a broadcast television camera, it is necessary that the chromatic aberration of the teleconverter lens be appropriately corrected.
In order to maintain high optical performance while downsizing the entire teleconverter lens, it is important to appropriately set a lens structure of the front lens unit having a positive refractive power and the rear lens unit having a negative refractive power. For instance, if the number of lenses constituting the entire system, the focal lengths of the front lens unit having a positive refractive power and the rear lens unit having a negative refractive power, and the materials of individual lenses constituting the individual units are not appropriately set, it is difficult to maintain good optical performance while downsizing the entire system.